Presumably they are..?

So – I can fit the rotor alone, which is sold as part of a 'front kit' with the front IS adaptor – on the rear, can't I?

Or am I missing something here..?

yes. no difference.

Look for front and rear markings

The discs would be identical, yes. The adaptor's can differ though, unless your talking a generic straightforward +20mm / +40mm type like A2Z, Hope etc.

Cheers, sweet! There's an inexplicable £32 difference between the 'Front Post Mount rotor kit' and the 'Rear IS rotor kit'. So I'm thinking, given I've already got the IS rear adaptor, I'll get the much cheaper front rotor kit and bin the post mount thingy..

if your talking about the caliper make sure the hose is long enough.

but rotors and mounts are interchangable.